Short-range order of crystallin proteins accounts for eye lens transparency

Abstract

In its normal state, the eye lens is transparent despite the presence in the cell cytoplasm of high concentrations of proteins, the crystallins, which, a priori, could be expected to scatter an important part of the incident light. Early on, an explanation was sought in the spatial correlations between individual scatterers. Trokel first proposed that the "high concentration of proteins in the lens must be accompanied by a degree of local order approaching a paracrystalline state"; Benedek subsequently suggested that a dense, noncrystalline packing of the proteins would sufficiently reduce the scattered intensity. However, in spite of an improved understanding of the molecular structure of crystallins, their spatial order remained unknown. We present here a small-angle X-ray scattering study of the problem, performed with calf lens cytoplasm both in intact lenses and in cytoplasmic extracts where the crystallin concentration was varied from 3 to 510 mg ml-1. All our experimental data are consistent with short-range spatial order, as in dense liquids or glasses, and this provides a simple explanation for lens transparency. In addition, we detected no conformational change or reorganization of the crystallin proteins throughout the investigated concentration range.